1. Field of the Invention
The present invention relates to a stiffening device (a flexible tower bar) used with a vehicle, more particularly, used with a vehicle having left and right shock absorbers to stiffen (improve the rigidity of) a vehicle body.
2. Description of the Related Art
Providing a beam-shaped stiffening device between vehicle body side attachment portions of left and right suspensions in the vehicle body of a vehicle such as an automobile is known in the related art. Among such stiffening devices, a device that is provided between left and right strut support portions of a strut-type suspension is known as a strut tower bar, and is widely used due to the ease with which it can be attached latterly to a pre-existing vehicle body and its effect of improving steering stability.
Providing a rotation allowing portion such as a pillow ball joint on an intermediate part of such a strut tower bar is also known in the related art (see Japanese Patent Application Laid-open No. 2006-182133, for example).
When a vehicle turns, a suspension arm on a turning inner wheel side tends to be drawn out toward a turning center side by the flexure of a vehicle body, including a sub-frame and so on, which is caused by a cornering force generated by a tire. In the case of a vehicle (a forward-projecting knuckle arm vehicle) in which a steering tie rod for steering a front wheel is disposed further forward than a kingpin axis serving as the steering axis of the front wheel, when the suspension arm on the turning inner wheel side is drawn out, a reduction (return) in the actual steering angle occurs, leading to a reduction in the slip angle of the tire and a reduction in the cornering force. Thus, the cornering force generated by a turning outer wheel increases relative to the turning inner wheel, thereby suppressing a jack-down phenomenon of the turning inner wheel side suspension and promoting a jack-up phenomenon of the turning outer wheel side suspension. As a result, rolling accompanying front rising pitching behavior occurs on the vehicle. When a vehicle turns, front falling pitching behavior is usually preferable in terms of steering stability and driver feeling, and therefore displacement of the suspension arm at the turning inner wheel side is preferably reduced to a minimum.
It is known that when the strut tower bar described above is attached, on the other hand, flexure of the vehicle body is suppressed, and therefore displacement of the suspension arm at the turning inner wheel side at an initial steering stage is suppressed. However, to achieve further improvements in driver feeling during a turn and steering stability in the initial steering stage, it is necessary to reduce displacement of the suspension arm at the turning inner wheel side even further to prevent return of the actual steering angle of the turning inner wheel.